Process of casehardening ferrous articles



Patented Feb. 25, 1930.

PATENT OFFICE PERCY A. E. ARMSTRONG, OF NEW YORK, N Y.

PROCESS OF CASEHARDENING FERROUS ARTICLES No Drawing. Application filed October 6, 1928,

It has heretofore been suggested that ferrous articles and ferrous alloy articles can be surface hardened by treating them with ammonia. In such process the articles are heated and apparently the ammonia decomposes and a certain amount of the nitrogen goes into solid solution in the surface of the metal and also tends to form nitrides. The action is, however, a reversible one, and while nitrogen is going into the metal, itv appears that a certain amount of the nitrogen is continually recombining with the hydrogen and being withdrawn from themetal. The penetration of the nitrogenis somewhat greater than that of the hydrogen, though the tendency of the hydrogen to re-combine with the nitrogen increases as the temperature is raised, so that if the action is conducted at increasing temperatures up to 1500F., relatively little nitrogen will be found in the ferrous articles, particularly near the surface As a matter of fact, when ferrous articles are treated with ammonia, one almost invariably finds a thin surface layer from which the nitrogen has largely been removed. and which is relatively soft below which appears the true hard layer.

I have found that the action of nitriding ferrous articles or ferrous alloy articles can be greatly accelerated if the article to be treated is exposed at elevated temperature to the action of the decomposition products re-' sulting from contacting ammonia with a heated mass of solid material having an affinity for hydrogen preferably in the presence of a catalyzeradapted to accelerate hydrogenation. For example, the ammonia gas may beled through or over a heated mass of pulverized carbon mixed with granules of nickel. In such case the ammonia is decomposed and the hydrogen combines with the carbon due to the catalytic action of the nickel, leaving nascent'nitrogen which penetrates the metal. The hydrogen appears to combine with the carbon to form methane and other compounds and it is my belief that these compounds have some effect which may be termed catalytic in assisting in the nitriding action. Presumably the hydrogen compounds formed are saturated or approximately saturated and substantially no free Serial No. 310,928. Renewed October 31, 1929.

hydrogen is present so there is very little tendency for hydrogen to recombine with the nitrogen previously absorbed in the metal. I also find that at the temperatures given,

there is no detrimental deposit of carbon on the metal. In other words, I carry on my nitriding process by subjecting ferrous articles to the action of nascent nitrogen in an atmosphere containing substantially no free hydrogen and which does not decompose in the presence of the iron to deposit carbon. lVliile some detrimental amounts of carbon dust may be carried mechanically to the articles to be'treated, it is desirable tokeep down the contact of carbon with the metal as much as possible, particularly where high tempera-- tures are used.

In carrying out my process, the ferrous arti cles to be treated are put into a closed container and the material or substance having an affinity for hydrogen is either'put in the same container, preferably in the path of the entering gases, or is put in a connected container through which the ammonia is passed.

The temperature used will depend upon various factors. If speed is the only consideration, a temperature up to 1400 F. may be used, because the absorption of the nascent hydrogen by the carbon or other substance used for this purpose will largely prevent the removal of previously absorbed nitrogen from the surface of'the metal, and an exceedingly hard strong surface will be obtained. The only important objection to the use of such a high temperature is that it may cause .85 the metal to warp and twist so that it is ordinarily advisible to use a lower temperature and a longer time,'which will give substantially identical results as far as surface strength and hardness are concerned. A minimum temperature is about 550 F. and a good operating temperature is between 825 and 925 which will give excellent speed to the absorption and will not cause twisting or warping of the pieces being treated. The 9 lower temperatures also serve to prevent any extensive absorption of carbon, though this docs notoccur to any great extent below a temperature of about 1200 F. If. there is any substantial amount of carbonization of the metal the process of 'nitriding will be greatly slowed up or almost entirely prevented.

If the solid contact material used to decompose the ammonia is in a separate centainer from the articles to be treated, this container also must be heated. The tem perature need not be as hi h as in the treating chamber but should be at least about 400 F.

The time of treatment will depend somewhat upon the temperature used and the depth of the nitride layer to be formed, but in any event it will be found that the time required is very much less than that necessary for an ordinary ammonia treatment. For example satisfactory results can usually be obtained in from 8 to 40 hours. If desired. the gases may be put under pressure which will further out down the time, but ordinarily the use of substantial pressures will not. be profitable due to the added expense of equipment required.

While my process may be used on various ferrous articles, I prefer to apply it'in cases of ferrous alloys which contain elements that combine well with nitrogen, and which preferably appear in solid solution in the iron, though in some cases I may use alloying ingredients which tend to form carbides: for example, I may use alloys of chromium, vanadium, titanium. boron, tantalum. molybdenum and aluminum in var ing proportions. A number of alloys w ich are particularly adapted for use with the present process are set forth in my co-pending application. Serial No. 38.170. filed June 19, 1925, and particularly the alloys therein dleseribed in which the alloying elements (including carbon) are toward the low side of the limits given. It is to be noted that because of this process I do not depend upon the carbon content of the alloys to produce hardness: in fact, the higher the carbon content of the ferrous article (which expression is intended to embrace alloy ferrous articles) the less will be the'action of the nitrogen. Accordingly carbon is to be employed in such ferrous articles merely to aid'in the control of the physical properties of the core rather than to give hardness to the surface. In other words, in order to obtain uniform nitriding the surface of the ferrous product should be of substantially uniform carbon content. and low to medium carbon content is preferable.

The nitride case hardening formed by my process is very resistant to ordinary corrosion such as that of the atmosphere or salt water or some weak acids, or salts such as copper sulphate, and does not materially lose its hardness even when reheated to a temperature equal to or slightly higher than that to which it was heated during the treatment with nitrogen; and the. hardness obtained by my process on ferrous alloy articles is considerably greater than'that of the hardest known hardened steel.

Where scale resisting ferrous alloys are used, such as those containing aluminum under 5%, but in substantial amounts, and chromium under 15% and preferably under 12% but in substantial amounts with or without other metallic elements, and the articles formed therefrom are case hardened in accordance withthis invention, it may be desirable to protect those surfaces which will later have to withstand the scalin efi'ect of subsequent reheating so that suc faces will not be subjected to the action of the nitrogen. This can be done by covering such surfaces with a cement or layer of nitrogen-impervious material; if preferred, the nitrogen treated layer may be removed as by grinding to leave untreated surfaces .to be exposed to such heat action. Thus, a valve for internal combustion engine of an alloy such as is disclosed in my co pending appli: cation, Serial No..38,170, may be hardened all over by treatment with nitrogen and then the surface under the head and on the bevel may be round sufiiciently to expose the nietalwhich has not been acted on by the nitrogen. This, however, is somewhat expensive and it is usually'desirable to preventthe access of nitrogen. In the same way threaded surfaces are preferably protected against the action of nitrogen.

Articles ease hardened by my process may be normalized by heating as for example to a temperatlu'c of 1000 F., followed by slow cooling and the surfaces that are to be treated should be freed from oxide, as by grinding or machining, or by any other process of cleaning thesurface. Usually the article can be produced in finished form before being subjected to the nitrogen treatment, as this treatment at the lower temperatures given, does not warp, twist or distort the article or change its shape except that there is a. slight volumetric change arising from the addition of the nitrogen. It is preferable surto heat the article in the presence of the decomposition products of the ammonia gas and then after'the treatment has gone sulficiently far, to allow it to cool down in'the gas atmosphere so as to'prevent oxidation during cooling. To prevent the access of air, the gas outlet from the retort in which the treatment is conducted is advantageously water, trapped. The water trap will also serve as a visual flow of gas. I

The operation ordinarily will becarried on in containers which together with the leadin and outlet pipes may be made of a metal as nickel, or highchromium silicon alloys such as are disclosed in my Patent No. 1,322,- 511. In packing the articles in the containguide for controlling. thegeously be used to insure a complete surface envelopment by the gas during the treatment. The articles which have been treated by my processmay be subjected to various heat treatments, but preferably such treatments are to be conducted at temperatures lower than 1500 F. However, such heat treatments will not ordinarily be used as the particular advantage of the nitriding process is that it gives a reasonably tough and very hard surface at temperatures of 1125 or lower, so that warping and twisting will be avoided and this advantage will be lost if the treated article is later subjected to higher, temperatures for heat treatment.

What I claim is:

1. In the process of nitriding ferrous articles, the step of decomposing ammonia in the presence of a substance having an aflinity for hydrogen and contacting a ferrous article at an elevated temperature with the resulting nascent nitrogen.

2. In the process of nitriding ferrous articles, the step of subjecting the articles to be nitrided to the decomposition products resulting from contacting ammonia with a sub stance having anaflinity for hydrogen, in the presence of a catalyzer.

8. The process of nitriding which comprises the steps of decomposing ammonia by contacting it with hot carbon in the presence of a nickel catalyst, and bringing the resulting decomposition products into contact with the material to be nitrided, at an elevated temperature.

4:. In the process of nitriding ferrous articles, the step of subjecting such articles at an elevated temperature below 1400 F. to the action of nascent nitrogen in the presence w of an atmosphere comprising hydrogen and carbon compounds but substantially free from free hydrogen or hydrocarbons which will decompose at such temperature in the presence of iron to deposit free carbon on 5 such articles.

PERCY A. E. ARMSTRONG. 

